Method and flux for soldering chromium oxide coated steel members



April 4, 1964 K. SCHUSTER ETAL 3, 4

METHOD AND FLUX F OR SOLDERING CHROMIUM OXIDE COATED STEEL MEMBERS Filed Feb. 1. 1960 mvsmoxs Ludw'ghischulsterak Alfanso L.Ba.Ldz3,Jn

ATTORNEY 5 United States Patent Ofiice 3,128,546 Patented Apr. 14, 1964 sylvania Filed Feb. 1, 1960, Ser. No. 6,021 3 Claims. (Cl. 29495) This invention relates to the coating of metal, more particularly to improve the corrosion resistance of the metal as in the manufacture of storage and shipping containers, building panels, and the like. The present application is in part a continuation of prior applications Serial No. 587,388 filed May 25, 1956 (now US. Patent 2,911,332 granted Nov. 3, 1959), Serial No. 592,552 filed June 20, 1956 (now US. Patent 3,031,333 granted April 24, 1962), Serial No. 666,852 filed June 20, 1957 (now US. Patent 3,112,231 granted November 26, 1963), Serial No. 708,772 filed January 14, 1958 (now abandoned, Serial No. 738,648 filed May 29, 1958 (now US. Patent 3,053,693 granted September 11, 1962), and Serial No. 814,200 filed May 19, 1959 (now US. Patent 3,053,- 702 granted September 11, 1962), prior application Serial No. 592,552 being in turn a continuation-in-part of application Serial No. 433,698 filed June 1, 1954, now US. Patent No. 2,773,623 granted December 11, 1956, and prior application Serial No. 587,388 being also a continuation-in-part of applications Serial No. 278,481 filed March 25, 1952, now Patent No. 2,768,104 granted Octo ber 23, 1956, and Serial No. 458,668 filed September 27, 1954, now abandoned.

Among the objects of the present invention is the provision of novel compositions for preparing the above coatings as well as novel methods for uniting metals carrying these coatings.

The above as Well as additional objects of the present invention will be more fully understood from the following description of several of its exemplifications, reference being made to the accompanying drawings, wherein:

FIG. 1 is a perspective view with parts broken away of a can having a coating typical of the present invention; and

FIG. 2 is a graphical representation of soldering compositions that are suitable for use in the manufacture of cans such as that of FIG. 1.

The above prior applications describe the coating of corrodible metals, particularly plain carbon steels such as SAE 1010 steel with an in situ formed combination of hydrated chromium oxides containing about 20 to 60% chromium by weight, about 40 to 95% of the chromium by weight being trivalent, the remainder being hexavalent, and the coating weighing about to 200 milligrams per square foot of surface that it covers. As disclosed in application Serial No. 592,552, such plain carbon steels, if given a grain-boundary etch before coating, show much greater corrosion resistance.

A can for liquid detergent, lubricating oil, cofiee beans, or the like, can have its inner surface covered with the above coating to prevent rusting prior to filling as well as after filling with the above materials; the outer surfaces of the can can also have the same type of combined chromium oxide coating which provides an excellent substrate for subsequent lithographic coatings, varnishes, lacquers, enamels and other organic coatings.

Instead of applying the coating of the present invention to the can after it is manufactured or after it is filled and sealed, it is simpler to make the can from steel sheets that have the coating applied while they are being produced. This permits the coated sheets to be stored, if desired, and even shipped over substantial distances without requiring any supplemental treatment to prevent them from becoming unmarketable or unsightly as a result of corrosion.

The sheets are readily coated by first making sure they are very clean and then passing them through an aqueous solution of chromic acid and a reducing agent which is compatible with the chromic acid. This compatibility means that the chromic acid and the reducing agent, even though both are present in the solution, will not in the appropriate dilution react rapidly With each other and will not form a visible precipitate while the solution is being contacted with the steel.

The steel sheet wet With the solution is then dried at a temperature above 212 F. During this drying, the chromic acid reacts With the reducing agent and becomes partially reduced. As a result, the final coating is a combination of hydrated chromium oxide containing about 20 to 60% chromium by weight.

An inordinately high degree of corrosion resistance is obtained when 40 to by weight of the chromium in the final coating is in what corresponds to the trivalent condition, the remainder being hexavalent, and the coating itself weighs about 10 to milligrams for every square foot of surface that it covers. Coatings of less weight give less protection, although coating weights need not be more than about 30 milligrams per square foot to give all the protection that is needed.

If the reducing agent is omitted from the coating bath, dried coatings of the above Weight range will not show the desired trivalent chromium content. They also tend to be somewhat deliquescent and therefore inferior.

Typical reducing agents suitable for use in connection with the present invention are organic polyalcohols such as sugars, including invert sugar, sucrose, dextrose, glycol and polyethylene glycols, glycerine, mannitol, sorbitol, triethanolamine, hydroxylamine salts such as its sulfate and hydrochloride, phosphorous acid, and others such as the aluminum lactate mentioned in application Serial No. 738,648. The preferred reducing agents are those like the polyalcohols which are oxidized by the chromic acid without leaving water-soluble reaction products. Phosphorous acid is oxidized to phosphoric acid by the chromic acid, and the phosphoric acid is converted to phosphates that are insoluble in water.

The proportion of reducing agent to chromic acid should be insufiicient for the complete reduction of all the chromic acid to trivalent condition. The minimum amount of reducing agent is somewhat below that which will in a body of solution reduce the lowest proportion of the chromium, inasmuch as some of the chromium is reduced during the drying operation even if the reducing agent is not present in the bath. Chromic acid solutions of almost any concentration can be used and the coating weight adjusted by controlling the amount of solution that is left on the metal surface When it is being dried. However, concentrations of from about /2 to about 30% CrO are most readily used.

The final heat treatment is somewhat more effective if carried out at temperatures substantially above 212 F. Temperatures between 275 F. and 450 F. are preferable and provide the most corrosion-resistant and adherent forms of coating. At about 500 F. and higher, however, the coating appears to be adversely affected unless the exposure to that temperature is held down to about tw seconds or less.

Referring now to FIG. 1, there is shown a can 10 representing one embodiment of the present invention. The can has a tubular body 12 made in the conventional manner from a single strip of a full bright finished steel 10 mils thick with the ends joined together as indicated at 14. These ends are folded back on each other to provide interlocking flanges 16 and 18 crimped together and soldered to make a suitably sturdy and leak-proof joint.

The openings at the top and bottom of the tubular body are sealed with covers 20, 22. that have beaded peripheries 24 with interlocking flanges 26, 23. The interlocking beaded periphery can be crimped and soldered or can be provided with a layer 30 of gasketing and merely crimped to assure hermetical sealing.

As shown by the broken-away edge 32, the metal of the body has a coating 34 applied in accordance with the present invention. This coating was provided on the unfabricated sheet by subjecting it to the following operations:

Example 1 (A) Clean the blackplate cathodically in an aqueous solution containing 16 grams KOl-l per liter using a current density of 15 amperes per square foot of cathode at 140160 F. for seconds.

(B) Cold water rinse.

(C) Clean anodically in an aqueous solution containing 16 grams KOH per liter using a current density of amperes per square foot of anode at 140160 F. for 10 seconds.

(D) Cold water rinse.

(E) Flood with a passivity-preventing /2% aqueous H 50 solution by weight 1-2 seconds at 80 F.

(F) Cold water rinse.

(G) Spray with an aqueous solution containing 2% nitric acid, 80 F., 8 seconds using a spray pressure of about 6 pounds per square inch.

(H) Cold water rinse.

(I) Desmut by brushing in water to remove loose or non-adherent reaction products including any developed in G.

(1) Flood with an aqueous solution containing 4% chromic acid and 1.3% cane sugar at 75 F. for 2 seconds.

(K) Roll through rubber rolls wetted with the flooded solution.

(L) Cure by passing the resulting filmed metal through a drying unit having a set of ceramic gas burners heated red hot by burning gas, a five second exposure to the incandescent units being used, and the metal reaching a temperature of 300 to 350 F.

The addition of a wetting agent such as 0.005% of the product made by condensing 3 mols of ethylene oxide with p-(n-octyl)phenol, improves the wetting in step I.

if the metal is in the form of an elongated strip, it may be coiled up directly after step L.

The same coated sheet can be used for making both the top and bottom covers 20, 22 as well as the body 12 of FIG. 1, although in some cases different thicknesses of metal can be used in the different portions, so that separate coated sheets are required.

After the can body is secured together in the above manner, the cover on one end can be applied in any convenient manner such as the one generally used in the industry and shown in FIG. 1. The can can then be filled and the remaining cover applied with the usual precautions in the case where the contents have to be sterilized or heated, or kept in a special atmosphere such as under evacuation or superatmospheric pressures. The cans having the coatings of the present invention are particularly useful in storing such materials as dried foods, nuts, spices, dough mixtures, etc., as well as the abovementioned coffee and lubricating oil. Wet-packed foods or other materials such as the liquid detergent referred to above can also be stored in the cans of the present invention, but here it is desirable to have an organic covering layer (enamel, lacquer, varnish) applied over the coating of the present invention on the inside surface of the can. Animal foods such as dog and cat food, either dry or wet, are also effectively preserved in such cans, and the organic covering layer can be omitted from the outside surface of the can if desired.

The coatings of the present invention contribute a substantial amount of increased adhesion and wear resistance for such organic coatings as well as resistance to corrosion. Suitable organic layers are those usually referred to in the art as enamels, sanitary enamels or lacquers such as the oleo-resinous phenolic or vinyl resin varnishes. Particularly efiective forms of such organic type coating are described in US. Patents Nos. 2,231,407, 2,299,433, 2,479,409 and 2,675,334. Such top coatings will even further reduce corrosion as well as increase the wear resistance and lower the contamination of the can contents by the oxides.

The advantages of the present invention are contributed to any plain carbon steel, that is steel that contains no more than about 2% of alloying metals. They can have a carbon content varying from extremely low values, 0.05% or even less, to as much as 1.4% or higher. The phosphorus and sulphur contents can range from substantially zero up to several tenths of a percent. Generally phosphorus maxima are about 0.15% and sulphur maxima about 0.3%. These materials include the steels ordinarily considered as carbon steels (SAE 1010 to 1095), free cutting steels, plain carbon tool steels, including those that have up to several percent silicon, and casting metals.

The inorganic mixed chromium oxide coatings of the present invention is even further improved if after the final high temperature drying it is subjected to a quench that rapidly reduces its temperature at least about 25 F. Any liquid appears to be suitable for this purpose, and plain or tap water is very effective. The addition of 0.03 to 1.0% CrO in the quenching water still further improves its effectiveness.

The above coating techniques can be readily carried out either in a batch process or continuously. They may, for example, be added at the end of a standard sheet steel production line. In fact, the coating treatment of the present invention can be carried out with the steel moving at a relatively high speed through the necessary treating stations. Under some conditions, particularly Where the steel is moving very rapidly through a chromic acid bath, it is desirable to have a wetting agent present in the bath. This enables the bath liquid to more rapidly and uniformly wet the surface of the metal. Wetting agents of any type can be used so long as their wetting action is not completely destroyed by the oxidizing action of the bath. Anionic, cationic or non-ionic types of wetting agent used in amounts of about 0.001 to 0.2% by weight of the bath is effective to keep from developing coating irregularities apparently due to air bubbles trapped on the metal while moving through the chromic acid bath. A highly effective example of wetting agent is the polyoxyethylene ether of alkylated phenols such as those produced by condensing dodecyl phenol with twelve molecules of ethylene oxide. Reference is also made to US. Patents Nos. 1,970,578 and 2,085,706 for more specifically disclosed wetting agents that are suitable.

Another example of the coating process of the present invention is as follows:

Example 2 (A) Sheets of 14 mil thick SAE 1010 steel are cleaned by immersing for five seconds in a 180 F. aqueous solution of 5% disodium phosphate and 3% sodium carbonate.

(B) The cleaned sheets are rinsed with water at 70 F.

(C) Immerse the rinsed sheets in solution of sulfuric acid in water for one second at 70 F.

(D) Rinse in Water at 70 F.

(E) Subject the resulting sheets to a uniform action of jets of an aqueous solution containing 1.2% nitric acid at F. for two seconds, the jets impinging at a velocity of ten feet per second.

(F) Rinse the acid treated sheets in cold water at 70 F.

(G) Pass the sheets between rotating brushes having stainless steel bristles, to remove loose material.

(H) The rinsed sheets are sprayed with an aqueous solution of Il /2% chromic acid, 1% pantaethylene glycol and 0.1% of the wetting agent made by condensing 3 mols of ethylene oxide with p-(n-octyl)phenol. This solution was prepared in the mixing nozzle by supplying it with two separate streams, one being an aqueous solution of the chromic acid, and the other in aqueous solution of the remaining ingredients.

(I) The sheets carrying the above solution are passed through an air oven, the inside of which is held at 800 F., the surface of the sheets reaching a temperature of 375 F.

(J) The hot sheets are quenched with water at 70 F., and then permitted to dry.

The dried product is extremely resistant to corrosion, particularly if coated with an acrylate lacquer or even a thin film of methylmethacrylate resin. Other resins such as those made with ester-type waxes, carnauba wax for example, are also very effective:

The oxide-coated steel of the present invention is further protected against corrosion by applying to the coating a film of an oil such as a paral'iin or a glyceride oil. Thus mineral oil or palm oil can be applied in very minute quantities (0.5 milligram or more per square foot) as by conventional electrostatic coating techniques and enhances the corrosion resistance of the oxide coatings as Well as reduces friction to simplify fabrication operations such as stamping, bending, etc.

Inasmuch as the chromium oxide coatings of the present invention show unusually high resistance to chemical attack, soldering to such coated metal is made somewhat difficult. Ordinary fluxes are not effective, and even with special flux the removal of the coating to permit access of the solder is a relatively slow process.

The can body of FIG. 1 can be soldered together after being blanked to shape, by dipping the edges to be joined first in a flux and then in a bath of molten solder. This causes the flux to attack the coating and permits the solder to wet the immersed metal over the area of the joint. The solder-carrying edges are then washed as with water to remove residual flux, and the resulting sheet fed to a can-forming machine where the solder-carrying edges are joined to form interlocking flanges 16, 18, and these edges are crimped together and subjected to a heat treatment to liquefy the solder and hermetically seal the joint.

Suitable fluxes for the above purposes are mixtures of hydrochloric acid and ammonium chloride. Neither of these ingredients by themselves is suitable, but when mixed in certain proportions they will together provide the necessary fluxing action. The more concentrated the hydrochloric acid, the less ammonium chloride is needed. At extreme dilutions the acid will, however, not be suitable regardless of the proportions of the ammonium chloride that is added.

FIG. 2 indicates the effective range of flux formulations with tin-lead type solder. In this figure, curve 50 represents the range of hydrochloric acid concentrations with the minimum proportions of ammonium chloride that will be suitable. Thus, point 51 on this curve represents a hydrochloric acid concentration of 35%, that is a solution of 35 grams of dry hydrogen chloride in 65 grams of water. At this concentration the acid requires the addition of 0.15% of ammonium chloride by Weight. In other words, for each 100 grams of the concentrated acid, 0.15 gram of ammonium chloride is added. The ammonium chloride dissolves in the hydrochloric acid so that the final mixture is a clear solution.

As indicated at point 52, when the hydrochloric acid concentration drops to 17.5% by weight the minimum ammonium chloride addition is 0.26%. Point 53 represents 7% hydrochloric acid, at which concentration 0.42% ammonium chloride is needed. As the acid concentration drops toward 3%, which appears to be the minimum that is effective, the ammonium chloride proportion increases sharply. At 4% hydrochloric acid (point 54) for example, 1.5% ammonium chloride is needed.

The above mixtures can also have their ammonium .ehloride content increased without affecting the fluxing action to any appreciable degree. In FIG. 2 there is accordingly indicated a shaded area 60 which represents the above formulations with the excess ammonium chloride that can be added. For convenience, the excess ammonium chloride is desirable since it reduces the criticality of the flux formulation. Suflicient excess ammonium chloride can be added to make a solution saturated with respect to this ingredient. If desired, even more ammonium chloride can be present, in which case it should not be present in such amount as to keep the flux from losing its fluid character. In view of the above, the upper limit of ammonium chloride formation has not been sharply defined in FIG. 2. For practical purposes it can be considered as that amount which will dissolve in the acid;

Although a 35% hydrochloric acid makes a suitable flux ingredient, as indicated above, it is usually desirable for commercial operations to use an acid that is no higher than about 25 to 30%. This reduces the amount of hydrogen chloride fumes that are evolved by the flux and makes it simpler to handle with less precaution needed to keep hydrogen chloride fumes from being spread around and attacking metal or other materials or personnel that might be nearby. This is particularly important when the flux-containing metal is dipped into the fused solder bath, inasmuch as the temperature of this treatment is more likely tocause the liberation of hydrogen chloride fumes.

Soldering is also a good way to seal joints in metal cans or the like where only one surface of the metal carries the mixed chromium oxide layer of the present invention. For example, a dog food can may have its internal surface tinned and its external surface coated with such oxide layer. The fiux treatment does not affect the tinned surface so that the attraction of the tin to solder helps in the soldering. The inside and outside surfaces of such a container can also be interchanged, but in either case it is helpful to apply an organic coating over the tin and/or the mixed chromium oxide coating, to improve their characteristics.

The soldering of the body can also be effected after the joint is prepared by folding and interlocking the flanges 16, 18. The flux and hot solder is in this case applied to the joint area after the interlocking, and the Washing off of excess flux can be accomplished immediately afterward or subsequent to the can sealing.

The removal of the mixed chromium oxide from the solder locations is also accomplished by an abrading operation or with hot strong aqueous sulfuric acid, from about 20 to 60% by weight and at a temperature of at least about F., or by masking off these locations during the coating step.

The soldering of container joints is helpful in that the solder covers and further protects the portions of the container walls that are crimped together to make the joints. Such crimping can be quite severe and can subject the metal to so much distortion as to fracture or loosen the mixed chromic oxide coating, particularly if the coating is relatively heavy. Soldering at a joint over such severely deformed metal will fully protect the metal against corrosion.

The coatings of the present invention can be applied to articles after they are formed as by shaping, stamping, or even casting. When treating such formed articles with the liquid coating solutions, however, care should be taken to see that the layer of solution on the article does not concentrate as by running or dripping to any portion of the article that should be protected. A localized thick layer of solution tends to form on the lower edges of the article under the influence of gravity, but a blast of air can be used to redistribute the thick layer. Alternatively, the articles can be rotated or kept moving so as to keep a thickened film from forming at any location.

Although it is indicated above that coating weights of at least ten milligrams per square foot are required to give the exceptional corrosion resistance for some purposes, such as to anchor organic covering layers, coatings of somewhat lower weight will provide unusually good results.

The weight of the coating is readily determined by dissolving it in the flux or in a strong alkaline solution such as 20% NaOH in water at 180 F. and weighing the metal before and after. The solution thus made can have its hexavalent chromium content determined by titration with sodium thiosulfate. The total chromium content can be determined by oxidizing another portion of the solution with sodium peroxide and then again titrating with sodium thiosulfate.

The coating solutions for the present invention need not be prepared from separate chemicals which have to be carefully combined in the proper proportions by final compounder. It is a feature of the present invention that the dry or concentrated chemicals can be mixed in the proper proportions and shipped as such a mixture for use in the coating operation. Such concentrated mixtures are more readily shipped since they take up less room and are not encumbered by large quantities of Water. Furthermore, these concentrated mixtures can be stored indefinitely and are actually more stable than solutions.

This stability is an unexpected characteristic of mixtures of CrO and sucrose, for example. When these materials are mixed together in perfectly dry condition they show no tendency to react, notwithstanding the fact that when in dissolved form these two materials react with each other more and more violently as the concentrations of the solutions increase. The regular commercial grades of these materials have moisture contents less than 0.1% and can be directly mixed, and the resulting mixture placed in a conventionally sealed container such as capped bottles or even paint cans with friction covers. The mixing need not be such as to produce a uniform mixture, although uniformity is readily obtained and is desirable where the entire contents of a single container need not be used at one time. However, the mixing itself should be carried out under conditions of low relative humidity or with only limited exposure to high humidity, in order to keep the CrO from absorbing too much moisture.

To use the prepared dry mixture for coating it is only necessary to drop the contents into a sufiiciently large quantity of water preferably with stirring, so as to prevent local high concentrations that can cause reaction. As soon as the solids are completely dissolved, the solution is ready for use. If undissolved material is present in the solution, it is preferably filtered before use.

Any of the above-mentioned water-soluble reducing agents that are solid will, when substituted for the sucrose, also be stable in the above mixtures and can be similarly used in conjunction with the present invention. All these reducing agents are compatible with a 4% water solution of CrO for at least one day at 80 F., but react with the CIO;, in films of said solution at temperatures over 212 F. to cause reduction from about 40 to 95% of the chromium in the CrO to trivalent condition. The proportions of the ingredients can be from 3 to 5 parts by weight of CrO as against from 1 to 2 parts by weight of the solid water-soluble reducing agent.

In accordance with the present invention the above dry mixtures can be further improved by including in them oxides or carbonates of metals such as zinc, magnesium, aluminum and calcium in a proportion up to that which is stoichiometrically required to form a dichromate with the CrO Such addition acts to dilute somewhat the dry concentration of the CrO and thereby further safeguards the stability of the dry mixture, should the mixture for example be inadvertently exposed to high humidity. Furthermore, these additions, particularly when sufiicient to actually reach the above stoichiometric proportion, greatly stabilize the coating solution that is eventually formed from the dry mix, all without detracting from the efficacy of the final coatings.

As an example of the above dry mixes, there can be combined 3 parts by weight of CrO one part by weight of sucrose, and 1.2 parts by weight of Zn(). The CrO is in the form of a powder that just passes through a mesh screen, the sucrose is in the form of ordinary granulated sugar as supplied to the grocery market, and the ZnO as a +250 mesh powder. The ZnO can first be mixed with the CrO after which the sucrose is added to give the final blend which can be stored for several years if necessary. Upon pouring into sufii-cient water to provide a solution having 3% CrO by weight, an etfective coating solution is prepared. In some cases a small amount of normal zinc chromate, insoluble in the solution, is also formed and it is desirable to decant or filter the solution away from such residue.

Instead of the above dry mixtures, a pre-prepared dichromate can be mixed with the reducing agents of the present invention. Strontium dichromate is an example of such a dichromate, and can be readily prepared as coarse crystals about A; to inch in size. In general dichromates of zinc, magnesium, calcium, strontium, and aluminum are suitable for this purpose and are in some respects more desirable since they can tolerate a somewhat higher moisture content than the mixtures with unreacted CrO and can in fact be mixed in atmospheres having a humidity of as much as 75% or more. It appears that these dichromates are less hygroscopic than CrO and not only is less precaution needed for the mixing, but the mixture need not be so carefully sealed.

The reducing agents can also be partially or completely protected from contact with the CrO or the dichromate. One simple way to do this is to introduce the ingredients into a container in such a manner that they do not mix to any appreciable degree. There will then be a fairly thin boundary layer between the ingredients where the only actual contact of the reactants will occur. The reducing agent particles can also be first thoroughly mixed with very fine powdery forms of the above zinc, magnesium or aluminum oxides, so that the particles tend to become coated by these oxide powders and thereby some- What insulated from the CrO As described in copending application Serial No. 666,- 852 filed June 20, 1957, Serial No. 708,772 filed Jannary 14, 1958, Serial No. 738,648 filed May 29, 1958, and Serial No. 814,200 filed May 19, 1959, the coatings of the present invention can also contain hydrophobic resins and/or pigment. These ingredients are provided in a form in which they are dispersed in water so that they readily mix with additional water to form the final coating solution. Concentrated resin dispersions having as much as 50% resin solids by weight are available, and in these concentrated dispersions the reducing agent of the present invention can be dissolved to form a stable concentrate which has compactness features approaching those of pastes. Such a formulation can be readily dissolved in water to the required dilution, and then mixed with the necessary solution of CrO or dichromate to form a coating solution that applies a resin containing coating as described in the last mentioned prior application. An illustration of such a concentrate is as follows:

Example 3 One gallon of an aqueous hydrophobic resin dispersion made as described below and containing 46% of solids by weight.

One gallon of a 50% by weight solution of sucrose.

The two liquids are mixed and yield a viscous stable milky dispersion that can be kept indefinitely and will, after dilution to 4% solids weight and in combination with an equal volume of a 6% CrO solution in water added in the form of ZnCr O provide a coating bath that when applied with rubber-covered rolls gives a very effective coating on AISI types 400 and 300 stainless 9 steel, aluminum, and galvanized steel, with a second cure at 400 F.

The above resin dispersion is made by mixing 3300 milliliters of deionized water,

200 grams of para-normal octyl phenoxy octaethoxy ethanol,

1000 grams of inhibitor-free methyl methacrylate,

800 grams of inhibitor-free ethyl acrylate,

36 milliliters of a solution of 0.3 gram FeSO -7H O (reagent grade) in 200 milliliters deionized water,

9 grams ammonium pcrsulfate.

This mixture at room temperature is placed in a glass container and stirred while there is added 9 grams sodium metabisulfite, 2.5 grams t-butyl hydroperoxide.

Stirring is continued until the temperature subsides after an initial rapid heat evolution. The reaction mixture is then cooled to room temperature and to it is added another batch of the same ingredients, all but the t-butyl hydroperoxide being dissolved in a quantity of deionized water A of that used in the original batch. The t-butyl hydroperoxide is then added with stirring to cause the second batch to react and bring the resin content to about 46%.

Other resins such as methyl methacrylate homopolymers, the alkyds, butadiene-styrene copolymers, polystyrene, polyvinyl chloride and even polytetrafluoroethylene, can be used in place of the resin of Example 3 with similar results. In general the proportion of resin to reducing agent should be between about 1:2 and 6:1 by weight, and the concentrates have at least about 30% nonvolatile ingredients (essentially resin plus reducing agent) by weight. Specific examples of using such other resins are given in copending applications Serial No. 708,772, Serial No. 738,648 and Serial No. 814,200.

It will be noted in this connection that the polytetrafluoroethylene resin does not form a uniform adherent resin film when used in accordance with the present invention, but that the final coating it makes can be wiped or brushed free of loose particles to leave a very light coating having very good corrosion resistance. Resins that are of waxy nature, such as polytetrafluoroethylene or even polyethylene, do not provide a coating suitable for receiving a covering layer of paint, for example, particularly when present in the coating at a concentration of as low as 25%. However, the degree of corrosion-resistance imparted by the resin-containing coatings whether waxy or non-waxy, is so high that additional protective films are not needed. On the other hand, when hydrophilic resins, such as polyacrylic acid, are used, the corrosion protection is much poorer.

Pigments can also be incorporated in the above concentrated resin dispersions, or where resin-free mixtures are desired, the reducing agents can be incorporated in the pigment dispersions of the type referred to for instance in copending application Serial No. 814,200. Efiective examples of such concentrates are:

Example 4 50 cc. of a 50% by weight solution of sucrose in water,

285 cc. of an arcylate resin prepared as described in Example 3 but with a ratio of methyl methacrylate to ethyl acrylate of 1:4 by weight,

167 cc. of an aqueous dispersion of a mixture of 95% carbon black and 5% phthalocyanine blue, the dispersion having a 25% solids content and stabilized with para-normal octyl phenoxy tri-ethoxy ethanol in a concentration of .05%.

The three ingredients are mixed together to form a stable dispersion that will keep without any of the ingredients settling out. Before use it can be first mixed with 100 cc. of an aqueous solution of zinc dichromate having a concentration of 1.05 grams zinc dichromate per 10 cc. and then diluted to 1 liter with water. The resulting mixture forms a very effective black coating on bare steel either plain or pre-etched with nitric acid, if the mixture is rolled onto the steel with rubber rolls and the thus coated steel heated with a gas flame to 400 F. for one and onehalf seconds.

Example '5 300 cc. of the acrylate resin dispersion similar to that of Example 3, sold by Rohm & Haas, Philadelphia, Pennsylvania, under its designation RHOPLEX C-72,

30 cc. of an aqueous solution of para-normal octyl phenoxy tetra-ethoxy ethanol having a concentration of 20% by weight,

cc. of a 50% by weight aqueous solution of sucrose,

200 cc. of an aqueous dispersion of phthalocyanine blue containing 25.6% pigment by weight and stabilized with 0.1% of the above para-normal octyl phenoxy tetra-ethoxy ethanol,

54 cc. of an aqueous dispersion of titanium dioxide containing 36% TiO by weight and 3.3% hydroxy ethyl cellulose.

The resulting mixture can be stored for indefinite periods although after some time some of the titanium dioxide will settle out. The settled material redisperses itself readily upon stirring.

Before coating there can be added to the mixture 300 cc. of an aqueous solution of zinc dichromate having 0.7 gram of the dichromate per milliliter, and it is then ready for application by a conventional spray gun. Aluminum construction panels so sprayed and then cured by holding for ten seconds in an air oven kept at 1000 F., show better outdoor resistance than anodized colored aluminum.

traight chromium steel, stainless steel, aluminized steel, galvanized steel and bare steel panels similarly coated are very satisfactory surface panels for building construction. Other pigments such as iron oxide reds, chrome yellows and chrome oxide pigments and the like, also can be used in place of the pigments of the above examples, or can be combined with other pigments to give varying colors.

Another pigment-free formula is as follows:

Example 6 1 gallon of Teflon dispersion prepared in accordance with Example III of US. Patent 2,478,229 and concentrated to about 55% solids by weight,

1 gallon of 50% sucrose by Weight,

30 grams of para-normal octyl phenoxy hexa-ethoxy ethanol.

The above mixture is stable and can be stored for many months. For use it can be diluted with twenty times its volume of water and the diluted material mixed with a 4 /2% solution of zinc dichromate in water. A zinc base die casting having a chromium plating over an intermediate nickel plating which is in turn over a copper plating, When dipped in the resulting solution and heated in an oven held at 390 F. until the metal reaches oven temperature, leaves a whiteish film. Wiping off the film with a cloth leaves an almost invisible residual coating layer that weighs about 3 to 6 milligrams per square foot and increases tremendously the corrosion resistance of the plated metal.

Some reducing agents such as those which are ionizable salts, tend to render dispersions unstable and are better avoided in the making of concentrated resinand/ or pigment-containing dispersions. The non-ionic polyalcohols such as sucrose, glycerine and the like do not show' this destabilizing tendency.

The corrosion resisting effect of the present invention is also obtained with brass, aluminum, copper, zinc, and other steels including 18-8 stainless steel, 17% chromium steel and high carbon piano wire.

The wetting agents used in the above examples are very effective in keeping the resin and/or pigment particles 1 1 dispersed, but they can be replaced by other non-ionic, anionic or cationic wetting or suspending agents without affecting the corrosion resistance of the final coating.

The resin-free and resin-containing coatings can also be applied by a tumbling barrel technique to objects that are small enough to be so treated. By way of example, small stampings for snap switches can be tumbled in a stainless steel basket while the parts are flooded with an aqueous solution of 4% zinc dichromate and 1% sucrose. The excess solution is permitted to drain off while the tumbling is continued, and with the tumbling still continuing', a stream of unheated air is blown through the basket. This is continued until all the parts are dry, after which they can be stoved to bring their temperature to 350 F. for best results. The parts are now ready for assembly into the switches. Plain carbon steel, alloy steels, stainless steels, brass, copper, aluminum, and in general all metals can be so coated. Heated air can also be used in place of unheated air if desired.

An efiicient tumbling basket for the above purpose is made of 18-8 stainless steel wire inch thick spaced inch apart, although other dimensions can be used for other types of work articles. Nylon-coated plain carbon steel wire can also be used in place of the stainless steel to make the basket. Ordinary perforated tumbling drums will work, but are somewhat less suitable.

Other coating solutions such as the dichromate-free CrO solutions and the resin-containing solutions can also be applied in the same manner.

Such objects as steel fuel lines for automobiles and the internal as well as external components of water meters, are also desirably coated with the resin-free or resincontaining coatings of the present invention. These fuel lines generally have an inside surface of bare steel which can become corroded by moisture present in the fuel. For treating these lines, the coating mixture of the present invention is introduced into the line after which a somewhat loosely fitted plug is forced through the line as by air pressure, to wipe off the excess solution. The sotreated line is then subjected to the high temperature operation. Where the plug is fitted too tightly, the concentration of the coating solution should be very high inasmuch as only very thin films of the solution will be left on the tubing wall.

In connection with the water meter use, the internal components of the meters are very effectively protected against corrosion for considerable periods of time even though these components remain immersed in water continuously during use. Such internal components made of stainless steel will also have their useful life greatly extended by such coating treatment.

A feature of the present invention is that unusually protective coatings can be applied from a water base mixture and in a single application. Fire hazards associated with organic solvents are completely avoided and the oneshot application is particularly desirable inasmuch as the curing step requires such a short treatment that it can be carried out on a rapidly moving metal producing line without calling for inordinately long ovens.

The proportion of chromium that is reduced to trivalent form in the coating of the present invention is preferably at least 70% of the total chromium present. Lower conversions leave the final coating with a brownish appearance and also do not give the best corrosion resistance. When the reducing agent is decreased in amount so that the chromium conversion drops below 40%, the curing time is increased and begins to become a serious obstacle. For instance, the complete elimination of a reducing agent from the resin-dichromate coating formulations produces coatings that are sensitive to moisture and do not provide sufiicient protection unless the curing is carried out for at least five minutes at a minimum of 350 F. Even then the chromium conversion is about 30% with a resin-todichromate ratio of about 2:1 only the C10 content of the dichromate being considered, and the conversion is even less with lower proportions of resins.

Such inadequately converted coatings are greatly improved by a hot water wash. For instance, a product coated as in the above Example 5 and cured for ten minutes at 325 F., develops better corrosion resistance and a more brilliant color if washed for one minute with 180 F. tap water. After such washing the chromium conversion analyzes at about 58%. For this improvement the washings should bring the conversion to at least 50% and generally at least a one-half minute treatment with water 160 F. or hotter is needed. The same improvement is brought about with all the resins and pigments.

The coating formulations of the present invention are especially effective when the hexavalent chromium is in the form of zinc dichromate. This dichromate is inexpensive as Well as very soluble in water and at the same time it shows an exceedingly low tendency to oxidize reducing agents when in water solution. It is accordingly the preferred form of hexavalent chromium.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed:

1. A method of uniting plain carbon steel members that have been coated with an in situ formed combination of trivalent and hexavalent chromium oxides, which method includes the step of soldering coated portions of the members together with a tin-lead type of solder, using as a flux a mixture of hydrochloric acid and ammonium chloride in which the range of proportions of ammonium chloride and hydrogen chloride is as represented by the shaded area of FIG. 2.

2. A method of making a storage container which includes the steps of providing a sheet of plain carbon steel coated with an in situ formed combination of hydrated chromium oxides containing about 20 to 60% chromium by weight, about 40 to of the chromium by weight being trivalent, the remainder being hexavalent, and the coating weighing about 10 to 200 milligrams per square foot of surface that it covers, and soldering the coated ends of the sheet together to make a tubular can body using a tin-lead type solder and a soldering flux in the form of a mixture of hydrochloric acid and ammonium chloride in which the range of proportions of ammonium chloride and hydrogen chloride is as represented by the shaded area of FIG. 2.

3. The method of claim 2 in which the soldering is effected by applying flux and solder to the ends before they are joined, then washing these ends to wash off residual flux, and then uniting the ends together and fusing together the solder on the united ends.

References Cited in the file of this patent UNITED STATES PATENTS 779,886 Stewart Jan. 10, 1905 1,939,467 Short et a1. Dec. 12, 1933 2,834,691 Stephenson et a1 May 13, 1958 2,927,046 Andrade Mar. 1, 1960 

1. A METHOD OF UNITING PLAIN CARBON STEEL MEMEBERS THAT HAVE BEEN COATED WITH AN IN SITU FORMED COMBINATION OF TRIVALENT AND HEXAVALENT CHROMIUM OXIDES, WHICH METHOD INCLUDES THE STEP OF SOLDERING COATED PORTIONS OF THE MEMBERS TOGETHER WITH A TIN-LEAD TYPE OF SOLDER, USING AS A FLUX A MIXTURE OF HYDROCHLORIC ACID AND AMMONIUM CHLORIDE IN WHICH THE RANGE OF PROPORTIONS OF AMMONIUM CHLORIDE AND HYDROGEN CHLORIDE IS AS REPRESENTED BY THE SHADED AREA OF FIG.
 2. 